1. Field of the Invention
This invention relates to cardiac pacers and, more particularly, to improvements in oscillator circuits used in cardiac pacers. While the invention will be described in most detail in association with demand type pacers, the invention is applicable to all kinds of pacers having an oscillator circuit therein which establishes the normal operating rate of the pacer.
2. Description of the Prior Art
In electrical heart pacers and in particular demand type pacers, electrical stimulating pulses are delivered to the patient's heart only in the absence of natural heartbeats. Generally, the demand pacer is designed to deliver an electrical stimulating pulse to the heart at a predetermined time interval after the last natural heartbeat, and to continue to deliver stimulus pulses at a fixed rate as long as no natural heartbeats are sensed by the pacer. If a heartbeat is sensed by the pacer during the timing interval of the oscillator of the pacing device, the pacer oscillator is reset so that it starts its timing cycle over again and the pacer output is inhibited so that no stimulus pulse is delivered to the heart. The time interval between the moment when the pacer oscillator is reset and the time when it completes a timing cycle is sometimes referred to as the escape interval.
Such demand type pacers are well-known, have been miniaturized, are usually self-contained and powered by battery and are now wholly implanted within the body. It will therefore be appreciated that the packing density of circuit components in such a pacer is very high resulting in conditions which could lead to circuit malfunctions as, for example, malfunctions caused by current leakage between adjacent components or leads interconnecting components. In the past, such circuit malfunctions have caused pacers to fail in a manner resulting in a rate runaway condition which is clearly undesirable for the patient. In fact, it could be lethal.
Accordingly, a primary object of the present invention is to provide in a cardiac pacer a means for limiting the pacer operating rate to a predetermined rate which, while above the normal pacer operating rate, is still considered to be a safe pacer operating rate.